The present invention relates to a thin film transistor which is used, for example, as a switching element for a picture element electrode in an active liquid crystal display element. The invention also pertains to a method for the manufacture of such a thin film transistor.
A conventional thin film transistor of this kind has such a structure as shown in FIG. 1, in which source and drain electrodes 12 and 13, each formed by a transparent conductive film, for instance, are disposed apart on a transparent insulating substrate as of glass and an amorphous silicon or similar semiconductor layer 14 is deposited on the substrate 11 between the source and drain electrodes 12 and 13. The semiconductor layer 14 is covered with a gate insulating film 15, on which a gate electrode 16 is formed.
The gate insulating film 15 is formed of an inorganic insulator such as SiN.sub.x or SiO.sub.2, and the gate electrode 16 is formed of a metallic material such as aluminum.
In general, the gate insulating film 15 of such an inorganic insulator is formed mostly by a plasma assisted CVD (Chemical Vapor Deposition) process. However, the plasma CVD process introduces difficulty in producing the semiconductor layer 14 of good quality because its surface is bombarded with high-energy particles.
Moreover, since the conventional thin film transistor has the structure in which the gate electrode 16 is protrusively provided on the gate insulating film 15, the surface of the transistor is relatively irregular. When such thin film transistors are used in, for example, a light crystal display element, gate buses are likely to be broken by the irregularity of the transistor array. In addition, the conventional structure requires, for the formation of the gate electrode 16, two manufacturing steps of forming a metal layer and then selectively etching it away.
As a solution to the problem involved in the formation of the gate insulating film 15 through the plasma CVD process, there has also been proposed a thin film transistor of the type employing an insulating organic molecular film as the gate insulating film 15. However, this thin film transistor also uses metal for the gate electrode 16, and hence still calls for the above-mentioned two steps for the formation of the gate electrode and suffers its protrusive structure.